Field of the Invention
The present invention relates to an image forming apparatus which forms an image by using an electrophotographic method, and, in particular, relates to an image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-function machine having functions thereof.
Description of the Related Art
In recent years, a color image forming apparatus such as a color copier or a color printer is increasingly demanded as an image forming apparatus of an electrophotographic type in the market. Further, it is demanded that the color image forming apparatus achieves an image forming speed comparable to that of a monochromatic image forming apparatus, image quality comparable to that of offset printing, more reduction in a size than before, a shorter interval for maintenance, and reduction in running cost.
In response to these demands, a function-separated developing device, in which a developing chamber for supplying developer to a developer bearing member and a stirring chamber for collecting the developer from the developer bearing member are separated, can be used.
A developing device uses, as developer, two-component developer having non-magnetic toner particles (toner) and magnetic carrier particles (carrier) in some cases. Such a developing device is widely used particularly in the color image forming apparatus, for example, because excellent hue is achieved since toner does not need to include a magnetic substance.
A function-separated developing device using two-component developer generally has a configuration as illustrated in FIG. 3 and FIG. 5. A developing device 400 has a developer container 41 in which developer is contained. The developer container 41 is divided into a developing chamber (developer conveyance path) 41a and a stirring chamber (developer conveyance path) 41b by a partition wall 410c extending in a vertical direction. A developer conveying and stirring screw 42 serving as a first developer conveying and stirring member and a developer conveying and stirring screw 43 serving as a second developer conveying and stirring member are respectively arranged in the developing chamber 41a and the stirring chamber 41b. At each end of a partition wall 41c in a longitudinal direction, each of openings (developer conveyance paths) 41d and 41e allowing passing of the developer between the developing chamber 41a and the stirring chamber 41b is provided. The first and second developer conveying and stirring screws 42 and 43 stir and convey the developer to circulate the developer in the developer container 41. A developing sleeve 440 as a developer bearing member is placed rotatably in the developer container 41 so as to oppose a photosensitive drum 1a. The developing sleeve 440 incorporates a magnet 45 as a magnetic field generating unit.
The developer is supplied from the developer conveying and stirring screw 42 to the developing sleeve 440. The developer passes through a gap between the developing sleeve 440 and a regulating blade 46 serving as a regulating member and the developer with a predetermined developer amount is supplied to a position opposing the photosensitive drum 1a in the developing sleeve 440. At this time, developer which could not pass through the gap exists on the developer container 41 side of the regulating blade 46. When an amount thereof is large, magnetic force by the magnet 45 in the developing sleeve 440 and force caused by rotation of the developing sleeve 440 cause great stress with respect to toner and carrier and a situation where deterioration easily occurs is brought. Thus, in order to reduce deterioration of the developer as much as possible, the magnet 45 by which a first magnetic pole and a second magnetic pole from the regulating blade 46 in the upstream side of a rotation direction of the developing sleeve 440 have the same polarity to reduce the amount of developer existing on the developer container 41 side of the regulating blade 46 can be used.
In a function-separated developing device 4, as illustrated in FIG. 3, as being close to the opening 41d through which the developer flows down, the amount of the developer existing in the stirring chamber 41a decreases and the developer in the stirring chamber 41a becomes unintentionally difficult to be supplied to the developer bearing member. Therefore, a supplying amount from the developer conveying and stirring screw 42 may lack so that density unevenness due to coating unevenness occurs. Further, when the magnet 45 is used for reducing the deterioration of the developer as described above, the density unevenness is more likely to occur.
A situation as described above is considered to become increasingly important as a printer or a copier using an electrophotographic method achieves higher speed and higher durability in recent years.
Thus, in an image forming apparatus of Japanese Patent Laid-Open No. 5-333691, described is a technique of sufficiently enhancing developer conveying ability of first and second developer conveying and stirring screws by increasing a rotation speed in order to stabilize coating property of a developing sleeve.
Further, in an image forming apparatus of Japanese Patent Laid-Open No. 2007-163811, described is a technique in which in order to achieve uniform height of a developer surface of developer in a longitudinal direction, a top surface of a partition wall serving as a bottom portion of a container is inclined such that an end of the bottom portion in a developer conveyance direction is higher than the other end.
However, the image forming apparatus in Japanese Patent Laid-Open No. 5-333691 has a situation that as the rotation speed of the developer conveying and stirring screws is increased to increase a developer conveyance amount, stress on the developer increases and a rotation torque increases, so that the rotation speed of the developer conveying and stirring screws is difficult to be increased largely.
In addition, in the image forming apparatus of Japanese Patent Laid-Open No. 2007-163811, at a part where the height of the partition wall is low, the developer falls down from between the developing sleeve and the partition wall, so that the developer is easily accumulated in a lower chamber serving as a second chamber. Thus, there is a situation of density unevenness due to drag, and further, occurrence of overflow of the developer due to inappropriate collection of the developer on the developing sleeve.